1. Field of the Invention
The subject invention is directed to a twist drill and in particular to the transition between the primary cutting edge (lip) and the secondary cutting edge of the drill point of a twist drill.
2. Description of Related Art
Twist drills are typically fabricated through grinding operations utilizing a grinding wheel moved about a blank in a predetermined pattern to create the relatively complicated geometries found on such drills. In particular, major surfaces of the drill point result through the fabrication of the flute and an associated gash adjacent to the flute.
Directing attention to FIGS. 1-3, a typical prior art drill 5 has a body 10 extending along a longitudinal axis 15 and a drill point 20 extending from the end 12 of the body 10. At least one flute 25, formed with a first grinding operation, extends along the body 10 and terminates at the drill point 20. The intersection of the flute 25 with the drill point 20 creates a primary cutting edge or lip 30. The drill point 20 is further defined by a chisel edge 35 extending outwardly from the longitudinal axis 15 to a chisel edge outer end 40. Extending from the chisel edge outer end 40 is a secondary cutting edge 45. This secondary cutting edge 45 is formed through a second grinding operation which produces a gash 50 and not only forms the secondary cutting edge 45, but provides to the secondary cutting edge 45a positive axial rake angle. The drill 5 illustrated in FIGS. 1-3 has a pair of flutes 25, a pair of primary cutting edges 30 and a pair of secondary cutting edges 45. For clarity, only one side of the drill will be discussed and, therefore, only one set of elements will be discussed with the understanding that the other set of elements are symmetric with the first set of elements in the pair.
The lip or primary cutting edge 30 is produced through the same grinding pass used to generate the flute 25 and the secondary cutting edge 45 is produced with a second grinding pass which concurrently generates the gash 50. However, this creates a discontinuity 55 created at the intersection of the primary cutting edge 30 and the secondary cutting edge 45. As a result of this discontinuity 55, as the drill rotates in direction R, the discontinuity 55 contacts the workpiece and material accumulates about the discontinuity 55 to create an undesirable built-up edge. Not only does this built-up edge hinder the cutting operation but, furthermore, acts as a source of concentrated heat which over time thermally degrades the surface of the drill point.
Typically the primary cutting edge 30 may have an axial rake angle Y of between 0 and 10 degrees, while the secondary cutting edge 45 will have a positive axial rake angle which is dependent upon the helix angle of the flute. For a straight flute, the helix angle will be zero. Depending upon the relationship of these two cutting edges, the discontinuity 55 may be further exaggerated such that when the primary cutting edge 30 has a high positive axial rake angle and the secondary cutting edge 45 has a neutral or positive rake angle, the intersection of these two cutting edges may create an even more prominent discontinuity 55 susceptible to producing built-up edges.
A design is needed to continue to provide the benefits offered by the positive rake angle of the primary cutting edge and the rake angle of the secondary cutting edge but to provide such benefits without the discontinuity which is counterproductive to these desirable drill features.